1. Field of the Invention
The present invention relates to the racemization of optically active tartaric acid.
2. Description of the Prior Art
Tartaric acid exists as the optically active enantiomers, L-tartaric acid and D-tartaric acid, as well as racemic DL-tartaric acid and meso-tartaric acid. Naturally occurring tartaric acid is mainly L-tartaric acid.
L-tartaric acid is produced as a by-product from argol in the manufacture of the juice from the grape (wine). Further, L-tartaric acid is used as a food additive in refreshing drinks.
The tartaric acid produced by synthetic processes is DL-tartaric acid, and accordingly when L-tartaric acid is separated from the racemic mixture by optical resolution, the same amount of D-tartaric acid is produced as a by-product. D-tartaric acid is not approved as a food additive, and accordingly D-tartaric acid is of little value. In order to increase the value of D-tartaric acid, it is necessary to racemize the D-tartaric acid. It has been known to racemize optically active tartaric acid by heating the isomer in an alkaline solution or in an aqueous solution of the optically active tartaric acid. In this method, a large amount of meso-tartaric acid is produced as a by-product. In fact, the method is known as a method of manufacturing meso-tartaric acid. Of course, the formation of mesotartaric acid causes a decrease in the yield of DL-tartaric acid. Consequently, in order to increase the yield of DL-tartaric acid, it is necessary to prevent as much as possible the formation of meso-tartaric acid. A need, therefore, continues to exist for a method of racemizing D-tartaric acid without the formation of meso-tartaric acid as a by-product.